A draft specification for this new format is available, and I am greatly interested in your views about it. It is based on HTML and MicroXML syntax. As such it is designed to be consumed by browser-like applications. There is currently a custom element prototype client.
The objective of the format is to capture map semantics, such that a client can consume the format and present a map if desired.
You can find the draft spec here. If you would like to contribute, you are welcome to fork it etc., but I would be grateful if you joined the Maps for HTML CG, or WICG for substantive contributions.
I'm happy to answer questions here as best I can.